1. Field of the Invention
The present invention relates to an organic electroluminescent display (“OELD”) device and a method of fabricating the same and, more particularly, to a small molecular OELD device which is fabricated by using a mixture of a phosphorescent dopant and at least two hosts and a method of fabricating the same.
2. Description of the Related Art
As consumers want to get information faster and more precisely with the advent of information era, development on display devices which are light and convenient to carry and have fast processing speed has been rapidly carried out. Of these display devices, an OELD device is a self-light-emitting device which electrons and holes are recombined in an organic light emitting layer if a voltage is applied to an organic layer having the organic light emitting layer and so requires no backlight which is used in a liquid crystal display (LCD) device. Therefore, it has advantages as a next generation display device in that it is thin and light, has a simplified process, and has features of a response speed of the same level as a cathode ray tube (CRT), a low voltage driving, a high luminous efficiency and a wide viewing angle.
The OELD device is classified into a small molecular OELD device and a polymer OELD device according to a material of an organic light emitting layer.
The polymer OELD device may have a single layer structure comprised of an organic light emitting layer between an anode and a cathode or a dual layer structure which further including a hole transporting layer, and thus a thin OELD device can be fabricated. However, it has a disadvantage in that it is low in stability and has short life span compared to the small molecular OELD device. In case where one kind of material is employed as a light emitting material, the small molecular OELD device are low in color purity and luminous efficiency, and thus a host-dopant combination is used to increase the color purity and the luminous efficiency through an energy transfer. Here, the host which contains a carbazole unit as a host material, i.e., a small molecular material has a disadvantage in that it is easily crystallized by heat which may be generated during a device operation, destroying the device.
In the fabricating process of the OELD device, in case where one color device is fabricated, the OELD device having a polymer can be simply fabricated using a spin coating process. However, it is inferior in life span and efficiency to the small molecular OELD device. The small molecular OELD device is more excellent in device characteristics than the polymer OELD device but is fabricated through a complicated fabricating process using a deposition process.
In case of a full color device, light emitting layers which represent red (R), green (G) and blue (B) colors are patterned to implement a full color, and the patterning of the light emitting layers is carried out by an ink jet printing or laser induced thermal imaging (hereinafter will be referred to as “LITI”). The LITI can use a spin coating characteristic “as is” and so has high pixel inside uniformity even in case of a large-sized device. The LITI is not a wet etching technique but a dry etching technique, and so it can resolve a problem caused by a solvent which may lower a device life span, and it can finely pattern the organic layer. In case where a plurality of organic layers having at least a light emitting layer are stacked using a deposition process and a full color is implemented using a shadow mask in the small molecular OELD device, the light emitting layers which implement R, G and B colors are stacked and patterned. In such an instance, a device life span of the small molecular OELD device is more excellent than that of the polymer OELD device, since the multiple layers are formed using a deposition process, but a fabricating process is more complicated because the R, G and B light emitting layers are stacked on respective pixel regions using the shadow mask to implement the full color. In addition, in case of fabricating the large-sized device, the shadow mask may be sagged, leading to misalignment. Further, since a space between the masks may be contaminated, it should be frequently cleaned, which checks a size increasing of device and a mass production. As described above, the small molecular OELD device is excellent in device characteristics but is complicated in fabricating process compared to the polymer OELD device. In order to resolve the problems, a method of fabricating the small molecular OELD device using the LITI has been introduced, so that a device having excellent characteristics and large pixel region can be simply fabricated. As described above, however, in case where the small molecular OELD device is fabricated using the LITI, there is a problem in that the small molecular material is easily crystallized by heat.